1. Field of the Invention
The present invention relates to a metal oxide that can be used suitably as a piezoelectric material and to a piezoelectric element using the metal oxide.
2. Description of the Related Art
A piezoelectric material is used for a device such as an ultrasonic motor, a vibration sensor, an ink-jet printer head, a transformer, a filter or the like that utilizes a piezoelectric element as a piezoelectric material having electrodes. In addition, a piezoelectric material having ferroelectricity is also used for a device such as a ferroelectric memory.
A major piezoelectric material that has been used for a device contains lead. For instance, PZT (a product manufactured by Clevite Inc.) that is a solid solution of PbTiO3 and PbZrO3 is used as a typical piezoelectric material. Recently, however, there is a concern that lead exerts a negative effect on human bodies, and many countries have started to impose restrictions such as RoHS Directive on the use of lead in glass or high-temperature solder. Therefore, as a substitute for the existing material, piezoelectric materials used in various devices are also required to be a non-lead material that does not contain lead. However, there is a problem that most of non-lead piezoelectric materials that are presently developed have a phase transition temperature existing in a service temperature range, which leads to a problem such as insufficient insulation.
As a typical material among the non-lead piezoelectric materials, there is BaTiO3. As to the insulation property of BaTiO3, Ti has formal charge of 4+ and the number of d orbital electron is zero. Therefore, the band gap is as large as 3.2 eV, and it is proved that the material has good insulation property (M. Cardona, Phys. Rev. 140 (1965) A651.).
However, a crystal structure of BaTiO3 transfers from a rhombohedral system to an orthorhombic system, then to a tetragonal system, and further to a cubic system as temperature rises. The temperature range of the tetragonal system is narrow as between −5 to 130° C. In particular, the cubic system of 130° C. or higher becomes paraelectric to lose piezoelectric properties, which is a problem for practical applications. To solve this problem, a certain material is doped for adjusting the temperature range to each specific use, which may deteriorate piezoelectric characteristics as a trade-off.
Concerning other material system, for example, there is BiCoO3, which has a tetragonal structure in a Bi system of an A-site. The BiCoO3 has a large value of c/a as 1.27 and a stable tetragonal structure in a wide temperature range −250° C. or higher to less than 240° C., which widens the service temperature range of the device.
However, in the BiCoO3 structure, Co has formal charge of 3+, and d orbital is occupied formally by six electrons. Therefore, the band gap is small as 0.6 eV, and the insulation property is deteriorated (Yoshitaka URATANI, Tatsuya SHISHIDOU, Fumiyuki ISHII and Tamio OGUCHI, JPN. J. APPL. PHYS., PART1 44, 7130 (2005)).
It may be possible to adopt a method of doping other element for a purpose of improving the insulation property. However, this method may also deteriorate the piezoelectric characteristic simultaneously, so it cannot be an appropriate method. In addition, a piezoelectric material containing an alkali metal has a problem that it is vulnerable to environmental deterioration.
Considering the above-mentioned situation, it is necessary to provide a novel metal oxide that can be used as a piezoelectric material having high insulation property and a stable tetragonal structure in a wide temperature range.